It is necessary in the production of many machine parts to first bore a plurality of holes in a normally metallic workpiece and thereafter to thread these holes. Prior to these operations the workpiece can be cut from a larger piece and subsequent to them it can be subject to further operations, such as embossing or finishing.
It is standard in production-line machining to use presses at the various stages, adapted to normally also serve for functions like shearing or embossing. For tapping the taps are carried in respective rotary chucks or holders carried on a base plate fixed to one of the two press platens. The chucks are provided with respective drive gears or pinions that are interconnected, typically by a chain, so that they can all be rotated about their respective axes by a transmission having an input nut or screw that is centered on and rotatable about an axis perpendicular to the closing direction of the press. The other platen of the press carries a threaded member that fits with the screw or nut and that itself is nonrotatable so that as the two platens move together this straight-line movement is converted into a rotation of the chucks holding the taps. Such machines are limited to tapping in the normally vertical direction the presses operate, so that a workpiece that must be laterally horizontally tapped will have to be turned before entering the tapping machine. In addition the manner of driving the individual tap holders ordinarily precludes spacing them too closely, less than 40 mm on center.
As described in French patent No. 2,495,979 the chucks are each normally of two parts that can move relative to each other along the respective axis, but that are rotationally linked. The part in which the tap is fixed is biased outward, that is toward the workpiece, by a relatively soft spring. Thus the press closes at the tapping station to advance all the taps toward the workpiece, and pushes them into the respective holes. Meanwhile of course the taps are being rotated. Until each tap bites into the respective hole, the compression of the respective spring allows the base plate to continue to move forward, and thereafter the tapping proceeds at a rate determined exclusively by the pitch and rotation rate of the tap. As the press opens, the rotation of the taps is reversed. Hence the tap rotation rate must be set to be fast enough to complete the short stroke of the tapping operation but not so fast as to overrun the advance of the base plate on outward movement into the workpiece. On the other hand it must not be so slow that the tap advance rate is less than the press return rate to prevent binding in the hole and stripping of it on opening of the press.
It has in fact been learned and will be discussed in more detail below that the cycling rate of a tapping machine, that is the time it takes from a given starting position to tap a group of holes and then return to this starting position, is substantially slower than that of many other production-line machines. A shear or embossing machine can often cycle several times a second or at the slowest once per second, but a standard tapping machine cannot normally cycle even once in a second. The result is therefore a duplication of this equipment so that half the tapping is done at one location and the other half at another, or a slowing of the whole production line to the rate of the one tapping machine.